[ruby-core:114373] [Ruby master Feature#19832] Method#destructive?, UnboundMethod#destructive?

Issue #19832 has been updated by kddnewton (Kevin Newton).


I don't understand how you would go about statically detecting instance variable mutations. Consider

``` ruby
class Foo
  def bar = instance_variable_set(:@baz, 1)
end
```

is `bar` "functionally pure" or not? Is it "destructive"? Very difficult to say. Now do:

``` ruby
module Wat
  def instance_variable_set(name, value) = "wat"
end

Foo.include(Wat)
```

I don't think it's viable to do this statically, certainly not reliably.

----------------------------------------
Feature #19832: Method#destructive?, UnboundMethod#destructive?
https://bugs.ruby-lang.org/issues/19832#change-104120

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
----------------------------------------
I propose to add `destructive?` property to `Method` and `UnboundMethod` instances, which shall behave like:

```ruby
String.instance_method(:<<).destructive? # => true
String.instance_method(:+).destructive? # => false
```

One main purpose of using these classes is to inspect and make sure how a certain method behaves. Besides arity and owner, whether a method is destructive or not is one important piece of information, but currently, you cannot achieve that from `Method` or `UnboundMethod` instances.

The problem is how to implement this. It is best if this information (whether or not a method is destructive) can be extracted automatically from the method definition.

Unlike owner and arity, it may or may not be straightforward by statically analyzing the code. I think that, if a method definition defined at the ruby level does not call a destructive method anywhere within its own definition, and no dynamic method calls (`send`, `eval`, etc.) are made, then we can say that the method is non-destructive. If it does call, then the method is most likely a destructive method (it would not be destructive if the internally-called destructive method is applied to a different object. Or, we could rather call that a destructive method in the sense that it has a destructive side effect).

If doing that turns out to be difficult for some or all cases, then a practical approach for the difficult cases is to label the methods as destructive or not, manually. We can perhaps have methods `Module#destructive` and `Module#non_destructive` which take (a) symbol/string argument(s) and return the method name(s) in symbol so that they can be used like:

```ruby
class A
  destructive private def some_destructive_private_method
    ...
  end
end
```

or

```ruby
class A
  def foo; ... end
  def bar; ... end
  def baz; ... end

  non_destructive :foo, :baz
  destructive :bar
end
```

or

```ruby
class A
  non_destructive

  def foo; ... end
  def baz; ... end

  destructive

  def bar; ... end
end
```

When the method is not (yet) specified whether destructive or not, the return value can be `"unknown"` (or `:unknown` or `nil`) by default.

```ruby
String.instance_method(:<<).destructive? # => "unknown"
```




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